The roll bonding of expensive, corrosion resistant metals to steel or other cheaper substrate materials to form a clad composite material is well known. Such metal composite materials are used extensively in chemical, petrochemical and similar process plant to minimise the cost of such installations. This reduction in cost is achieved by using the cheaper substrate material to provide the composite with the required strength for structural requirements whilst the outer clad `veneer` of expensive metal is of a minimum thickness sufficient to provide the necessary corrosion resistance.
Such composites can be made by a variety of methods, the most frequently used being that of roll bonding. This method requires the surfaces of the cladder and substrate materials to be cleaned and placed in contact with each other to form a loosely assembled composite package which is heated to a suitable temperature before passing between rolls which compress the parts together and progressively reduce the composite thickness. The resulting deformation causes the cladder and substrate components to bond at their interface to form a bonded clad composite of reduced thickness and extended area.
Not all cladder materials can be roll bonded in this manner because some such metals are incompatible for bonding to the chosen substrate material. Two such examples are titanium and zirconium which cannot be roll bonded directly to a steel or stainless steel substrate because of the formation of deleterious intermetallic substances at the interface at the temperatures which are required to effect the bond. In such cases, the otherwise incompatible substrate and cladder components are normally joined by explosive bonding them together at the final required thicknesses. This process is not only relatively expensive but it also has technical limitations which restrict the area and thickness of composite which can be produced. To produce comparatively large areas, quality assurance procedures must be highly detailed and rigorously enforced by skilled supervision and, despite this, high levels of rejection of defective components can be experienced which cannot be re-worked. Consequently, expensive material and labour replacement costs are incurred which must be catered for by contingency costing.
Ingots have also been produced by explosive bonding cladder and substrate materials of appropriate thickness proportions. These ingots then being heated and `conversion rolled` to simultaneously extend the surface area and reduce the composite to the required overall thickness.
This technique is applicable to many cladder and substrate metal combinations and has also included titanium or zirconium explosively bonded to carbon and stainless steel substrates.
The more extensive use of this technique has been inhibited by the face that both titanium and zirconium, when bonded to these substrate materials, form brittle intermetallics at their bonded interface when heated to the high optimum temperatures necessary to provide the steel substrate with the mechanical properties defined in most pressure vessel specifications. These intermetallics form at temperatures above approximately, 850.degree. C. and effectively weaken or destroy the bond. Consequently, it is necessary that the conversion rolling of the explosively bonded ingot be carried out at temperatures below 850.degree. C. to avoid the formation of these intermetallics.
A further complication is that titanium also undergoes a phase change at approximately the same temperature that the growth of intermetallic occurs with the normal alpha structure of the titanium transforming to the beta phase. This has proved a further incentive to maintain conversion rolling temperatures below a temperature of 850.degree. C. Previous work in these lower temperature ranges, in which the titanium exists in the alpha condition and is maintained at a temperature below that where intermetallics are formed, is defined in U.S. Pat. No. 4,612,259. This patent discloses the use of two or more interlayer materials to prevent the growth of titanium/steel intermetallics and defines the rolling temperatures as being below 850.degree. C. and the selected interlayer materials include materials having melting points lower than the optimum rolling temperatures required to control the steel properties. Such materials will cause the titanium and steel substrate materials to separate due to the melting of these lower melting point interlayer materials if they are processed at the higher optimum temperatures necessary for adequate control of the steel properties.
Conversion rolling at temperatures of 850.degree. C. and below, hardens the steel to a greater extent, making compliance with the accepted steel specifications extremely difficult. In commercial practice, where larger ingots are rolled to extended areas, an initial lower ingot temperature below 850.degree. C., results in rapid cooling during the rolling to temperatures which require the reheating of the composite if the rolling is to continue without damaging the rolling mill and, consequently, any control of the steel properties is effectively relinquished. Such reheating also requires the use of additional furnaces which are, necessarily, extremely large to accommodate the greatly extended areas of the semi-rolled product.
Attempts have been made to control the mechanical properties of the steel, when rolling at temperatures below 850.degree. C. by modifying the composition of the steel. This normally entails a reduction of the carbon content to levels such that the level of work hardening which occurs at these lower temperatures is reduced to acceptable levels. Such modified materials, however, are almost invariably unsuitable for the other fabrication procedures which remain to be completed during the further construction of the vessels in which the composite is incorporated. Consequently, these techniques have not proved viable as a production process.
Attempts have also been made to facilitate the roll bonding of titanium and zirconium to steel by the use of interlayer materials which are compatible with both the cladder and substrate components. Such interlayers are designed only to effect the roll bonding of the component materials, and are necessarily comparable or below those of the cladder and substrate materials. Such techniques have not proved to be a commercial success and have had only limited market acceptance due to variable bond quality over the area of the product. The lower melting point of these materials again necessitates their rolling at temperatures below the optimum required for the control of the steel properties thus limiting their use to applications in which corrosion resistance is the principal acceptance criterion and the mechanical properties of the steel are not of overriding importance. The lack of uniform bonding over the entire area of the clad interface has given such products a reputation as having unreliable mechanical properties.
The present invention overcomes the previously described limitations as the use of appropriate high melting point interlayer materials prevents the formation of intermetallics between steel and zirconium and between steel and titanium despite any phase changes which occur in the materials at the high rolling temperatures required to produce the required mechanical properties in the steel substrate. When the composite is eventually cooled in the normal manner to temperatures below approximately 850.degree. C., titanium will revert to the original alpha phase condition. In the event that any titanium should remain in the beta phase form due to subsequent heat treatments required by the steel, for example quenching, this is of no consequence as the corrosion resistance of the titanium will remain unaffected.